The present invention relates to a technique for manufacturing semiconductor integrated circuit devices, and in particular to a method for manufacturing semiconductor integrated circuit devices using a phase shifting lithography technique in exposure processing.
In the photolithography technique of transferring a circuit pattern on a photomask to a semiconductor substrate by using a light beam such as the g-line (436 nm) or the i-line (365 nm), limits are being posed on minimum working dimensions of the pattern allowing favorable transfer as the degree of the device integration of semiconductor integrated circuit devices is improved.
It is conceivable to further shorten the wavelength of the exposure light beam as a method for making the minimum working dimensions of the pattern allowing favorable transfer shorter. As a matter of fact, however, there are various problems, and the wavelength of the light beam cannot be shortened simply.
Therefore, there is a technique of increasing the numerical aperture (NA) of the optical system in exposure apparatuses in order to improve the resolution without changing the exposure length. In this case, there was a problem that the depth of focus became excessively shallow with the increase of the NA and use of a short-wavelength light beam.
Therefore, various exposure techniques for attempting to improve the resolution without making the depth of focus shallow are under study. As the representative means thereof, there is a phase shifting lithography technique using a phase shifting mask.
The phase shifting lithography technique is a technique for improving the resolution and contrast of the projected image by manipulating the phase of the light transmitted through a phase shifting mask (including a reticle). In a predetermined position on the phase shifting mask, a phase shifter for generating a phase difference between transmitted light beams is formed.
For example, according to a phase shifting technique disclosed in JP-B-62-59296 (published on Dec. 10, 1987 and corresponding to U.S. Ser. No. 365672 filed on Apr. 5, 1982), a transparent film is disposed in one of a pair of transparent areas which are adjacent to each other with an opaque area between, so that a phase difference is generated between light beams transmitted through these two transparent areas, and the transmitted light beams interfere and weaken each other on a location corresponding to the opaque area on a semiconductor wafer.
According to a phase shifting technique disclosed in JP-A-62-67514 (published on Mar. 27, 1987), a part of the opaque area of a mask is removed to form a fine opening pattern and thereafter a transparent film is disposed either in the opening pattern or in the transparent area located near the opening pattern, so that a phase difference is generated between the light beam transmitted through the transparent area and the light beam transmitted through the opening pattern, and the amplitude distribution of the light beam transmitted through the transparent area is prevented from spreading in the lateral direction.
According to a phase shifting technique disclosed in JP-A-2-140743 (published on May 30, 1990), a phase shifter is disposed in a part of the transparent area of a mask so that a phase difference is generated between the transmitted light beams and a phase shifter boundary portion is emphasized.
In the case of the phase shifting lithography technique, its application to the transfer of a pattern including a simple repetition poses no problems. In the case where the phase shifting lithography technique is applied to the transfer of a complicated pattern such as a pattern for forming a semiconductor integrated circuit device, however, it is difficult to dispose phase shifters and in some cases a problem that the pattern cannot be transferred favorably is posed.
For example, as for mutually adjacent word lines of a DRAM, the space between adjacent word conductor lines including an area in which bit line contact holes and capacitor contact holes are disposed becomes wider on some locations than the space between other portions of those word lines or the space between other adjacent word lines because of the alignment tolerance of the contact holes.
In the case where such a pattern for the word lines is transferred by using a phase shifting mask, a phase shifter is disposed on either one of mutually transparent areas (for word line transfer). If there are portions having different spaces between adjacent transparent areas as described above, however, a difference is caused in strength of transmitted light beam between the portion and another portion. Therefore, the phase of the light beam can be manipulated favorably. As a result, a portion originally desired to be wide might become narrow, or a portion which need not be thick might become thick. Thus the shape and dimensions cannot be obtained as designed and the pattern cannot be transferred favorably in some cases.
Furthermore, in DRAMs for example, the memory capacity as a whole tends to increase and an increase in degree of device integration is attempted. As the degree of device integration is increased, the space of lines included in adjacent capacitor patterns is also narrowed.
In the case where a pattern for capacitor contact holes is transferred by using a phase shifting mask, at least one auxiliary pattern segment is disposed around a transparent area for forming contact holes. As the space of lines included in adjacent capacitor patterns is made narrower as described above, however, simply disposing auxiliary pattern segments might cause a pattern to be formed in a position corresponding to an area between auxiliary pattern segments, i.e., in an area in which a pattern should not be formed originally. This is caused by, for example, interference between light beams transmitted through adjacent auxiliary pattern segments.
In JP-A-6-130646 (published on May 13, 1994), a photomask having four rectangular phase shifters around a main hole has been shown.
In JP-A-6-289591 (published on Oct. 18, 1994), a phase shifting mask having auxiliary openings disposed on opposite sides of a main opening so as to be displaced from each other with respect to the center of the main opening and a phase shifting mask having a plurality of auxiliary openings disposed around the main opening at a predetermined pitch has been disclosed.
In JP-A-5-19446 (published on Jan. 29, 1993), a phase shifting mask having a predetermined auxiliary pattern disposed in a boundary portion of a pattern area has been disclosed.
In JP-A-6-85086 (published on Mar. 25, 1994), it is shown that problems such as shortcircuit caused between lines by a step are prevented by forming a contact pad having a line width smaller than the minimum line width so that the contact pad will have the same height as a flattening insulating layer and become flat.